Method of producing a biaxially oriented container of polyethylene terephthalate or similar material

ABSTRACT

A method for producing a container from a tubular blank of polyethylene terephthalate or similar material, according to which a polyethylene terephthalate blank is stretched at least approximately 3-fold to produce a preform (20). In an axial section through the preform, the contour length substantially corresponds to the contour length in an axial section through the final container. The preform (20) is heated to a temperature above the glass transition temperature (TG) and is converted to the container (21) in a blow mold (12). In doing so, the contour length of the material is maintained through successive reduction of the axial length of the body (23) which the preform constitutes during the conversion to the container. In order to achieve this, the base portion of the blow mold (11) is moved towards the orifice (22) of the final container. The container acquires a neck portion (23) and a container body (26) which are axially oriented, in the case of polyethylene terephthalate to an extent corresponding to about a 3-fold monoaxial stretching, while the container body (26) is also transversely oriented, in the case of polyethylene terephthalate to an extent corresponding to about a 3-fold stretching as a maximum. The wall thickness of the container is inversely proportional to the radius of the container. The container is dimensionally very stable and has excellent strength characteristics.

This application is a division of application Ser. No. 264,829, filedMay 18, 1981, now U.S. Pat. No. 4,467,929.

FIELD OF THE INVENTION

The present invention relates to a method for moulding a container,starting from a tubular blank of polyethylene terephthalate or similarmaterial, wherein, according to the invention, the blank is stretchedaxially, in the case of polyethylene terephthalate at least 3-fold, inorder to produce a preform, in which the material has a profile lengthwhich substantially corresponds to or exceeds the profile length ofthose parts of the container which correspond to the stretched part ofthe preform, after which the preform is heated to the mouldingtemperature and converted to the container.

PRIOR ART

In the production of containers made of a thermoplastic, where thematerial has a property that its strength and stability increase withorientation of the material, it is desirable that as large a proportionof the container as possible should consist of oriented material. Thematerial is oriented by a stretching procedure, with biaxially stretchedmaterial giving the best utilization of the available amount ofmaterial. Especially in the case of bottle-type containers it is,however, difficult to achieve stretching, and thereby createorientation, of, for example, the neck and orifice parts, and in certaincases even of the centrally located base parts.

Swedish Patent Application No. SE 8004003-3 and corresponding U.S. Pat.No. 4,405,546 disclose how axial orientation of the material in thepreform can be achieved by a mechanical procedure wherein a preformpasses through one or more draw rings which reduce the wall thickness inthe preform and thereby also the outer diameter of the preform. It hasbeen shown that the best effect is achieved when the material in thepreform, immediately prior to stretching, has a temperature within orbelow but near the region of the glass transition temperature (TG) ofthe material. The axially oriented preform obtained by the processdescribed is mechanically stable and the subsequent treatments forconverting the stretched preform to a container must therefore beappropriate to this fact.

Exceptionally good mechanical properties are achieved with polyethyleneterephthalate if the material, during stretching of the preform, isstretched at least about 3-fold. In this connection, an"over-stretching" of the material is easily achieved, resulting in thematerial in the preform shrinking during the heating of the preform tothe moulding temperature, so that the preform reaches a lengthcorresponding to that it would have reached on stretching about 3-fold.

On conversion of a preform, axially stretched as above, to a container,a problem arises in the form of fracture in the material if attempts aremade to stretch the preform additionally to the stretching which thematerial has acquired through the axial orientation of the preformdescribed above. During conversion of the preform to the container, onetherefore seeks to limit the axial stretching of the preform andessentially let the conversion result in a stretching of the materialonly in the circumferential direction of the preform.

In certain applications, a particular combination of container lengthand container diameter is sought but it is not possible to produce sucha container by known techniques because, during the conversion to acontainer, the material of the preform is stretched far too much in theaxial direction of the preform. This problem arises with a containerhaving a relatively large diameter in relation to the container length.Containers with such dimensional characteristics are normally involvedwith bottles having a volume of less than 0.5 liters.

SUMMARY OF THE INVENTION

The present invention contemplates, and a method for moulding acontainer, where the problems mentioned have been eliminated. Theinvention will be described especially in relation to polyethyleneterephthalate, hereafter referred to as PET, but is, in principle,applicable to many other thermoplastics of the polyester or polyamidetype.

Examples of such other materials are polyhexamethylene adipamide,polycaprolactam, polyhexamethylene sebacamide, polyethylene 2,6- and1.5-naphthalate, polytetramethylene 1,2-dioxybenzoate and copolymers ofethylene terephthalate, ethylene isophthalate and other similar plasticspolymers.

According to the invention, the material, in the form of a tubular blankof PET, is stretched at about least 3-fold to produce a preform having alength appropriate to the dimensions of the final container. The length,in the axial direction, of the stretched material of the preform isequal to or greater than the profile length of the parts which thestretched material constitutes in the container after conversion of thepreform to a container. The preform is then heated to the mouldingtemperature and is converted to a container, for example in a blow mold.As the preform has a length greater than that of the final container,hence greater than that of the internal height of the blow mold, it isnot possible, without special measures, to introduce the heated preforminto the blow mold. According to the invention, the conversion of thepreform to a container takes place so as to maintain the contour lengthof the material in an axial section through the body which is formedduring the progressive conversion of the preform to a container, thisbeing achieved through a successive reduction in the axial length of thebody in question.

In a preferred embodiment of the invention, the material, immediatelyprior to the stretching in the axial direction of the blank, is at atemperature in the range of or preferably below the glass transitiontemperature (TG) of the material.

The tubular blank is preferably stretched by passing it through one ormore draw rings so as to reduce the wall thickness of the material andat the same time diminish the outer circumference of the blank.

In an embodiment where the container has exceptionally high dimensionalstability at elevated temperature, the blank is so chosen that thepreform, on heating to the conversion temperature, suffers a reductionin contour length, in a section in the axial direction of the preform,to a value which is not below that of the contour length in an axialsection of the final container.

In one embodiment of the invention, the base of the blow mold is moved,during the conversion of the preform to a container, in the axialdirection of the preform and towards the orifice of the preform, inorder to assume a position in which the final inner shape of the blowmold is fixed.

In another embodiment of the invention, a central portion of material atthe base of the preform is reshaped and/or reduced in thickness throughthe portion of material being pressed between the base of the blow moldand a mandrel located inside the preform.

In a preferred device according to the invention, at least two andpreferably three parts of the mould can be moved to and fro from aposition where they cooperatively provide an inner shaping surface inthe blow mould. Two of the mould parts are mould halves, which can beopened and shut in the normal manner, to form the blow mold. The thirdmould part is the base part of the blow mold and can be moved in theaxial direction of the blow mold, so that the inner height of the blowmold can be varied.

Furthermore, a mandrel is used for fixing the preform, at its orifice,against two gripping halves, and for introducing excess pressure intothe interior of the preform.

In particular embodiment of the invention, the inner mandrel ends in ashaping surface which fits the shape of the base portion of theblowmould. When the base portion is in its upper position, the mandrelaccordingly cooperates with the base portion so as to reshape a centralportion of the base of the preform and/or reduce the thickness of thecentral portion of the base.

In yet another embodiment of the invention, the base portion of the blowmold is provided with a central mould portion which is thermallyinsulated from the rest of the base portion. The central mould portionpossesses channels for the transportation of liquid in order to controlthe temperature of the shaping surfaces of the central mould portion.The rest of the base portion is also provided with channels for thetransportation of liquid and control of the temperature of the othershaping surfaces of the base portion. Furthermore, all the other mouldportions, as well as the mandrel and the gripping halves, have channelswhich serve a corresponding purpose.

According to the invention, a container is formed which has a neckportion with axially oriented material and a container body which isbiaxially oriented, preferably with the exception of a central baseportion of the container body. The ratio of the material thickness inthe container body and the material thickness in the neck portion isapproximately equal to the ratio of the radius of the neck portion tothe radius of the container body in any plane at right angles to theaxis of the container.

In a preferred embodiment of the invention, the central portion ofmaterial at the base of the container and/or the orifice edge of thecontainer consist of thermo-crystallized, opaque and dimensionallystable material.

BRIEF DESCRIPTION OF THE DRAWING

The invention is described in greater detail in relation to four figuresof the drawing, in which

FIG. 1 shows a longitudinal section through a blow mold, with an axiallystretched preform placed in the blow mold, and with the base portion ofthe blow mould in the lower position,

FIG. 2 shows a corresponding longitudinal section during conversion ofthe preform, and with the base portion moving towards the orificeportion of the preform,

FIG. 3 shows a corresponding longitudinal section, with the base portionin its upper position and with the preform converted to the containerand

FIG. 4 shows a container produced in accordance with the invention.

DETAILED DESCRIPTION

FIGS. 1-3 show two mould halves 10 a,b, which are movable, in thedirection of the arrows A, B, to and fro from the position as shown inthe figures. The mould halves cooperate with a third mould portion inorder conjointly to form a blow mold 12, in which the third mouldportion 1 is the base portion of the blow mold 12. The base portion canbe moved between a lower position (FIG. 1) and an upper position (FIG.3) by means of drives (which are not shown in the figures). In the upperposition, the base portion, together with the mould halves, constitutesthe assembled blowmould.

The base portion is provided with a central mould portion 17 which isthermally insulated from the rest of the base portion. A number ofchannels 18 for the transportation of liquid are present in the centralshaping portion. Channels 19, serving a corresponding function, arelocated in the outer parts of the base portion, similarly to channels 30in the mould halves.

Furthermore, gripping devices 13 a,b are connected to the upper parts ofthe mould halves and cooperate with a mandrel 15 so as to hold a preform20 at its mouth or orifice 22. The gripping devices also possesschannels 31 for the transportation of liquid for temperature control.

The mandrel 15 has a length matching the upper position of the baseportion 11, so that with this base portion in its upper position a spaceis created, between a lower shaping surface 16 of the mandrel and thecentral mould portion 17 of the base portion 11, which space has a shapeand thickness determined by the intended shape and thickness of themoulded container.

Furthermore, the mandrel is provided with a main channel 14 for thepressure medium, which via the side channels 33 passes to the surface ofthe mandrel and hence to the interior of the preform 20.

In FIG. 1, the preform is placed in the blow mould and the centralportion 24 of material of the base of the preform is in contact with thebase portion 11 of the blow mold 12.

FIG. 3 shows a moulded container 21, and FIG. 2 shows a body 23 whichillustrates the preform being converted to the container 21.

FIG. 4 shows, in detail, a container according to the invention. Thecontainer has a neck portion 25 with an upper part which forms theorifice edge 22. The actual container body, 26, has in its base portiona central material portion 24. In the figure, the radius and materialthickness of the neck portion are designated r₁ and d₁ respectively,while r and d are the radius and material thickness of the containerbody in an arbitrary plane at right angles to the axis of the container.

The material thickness d varies in the container body with the radius rof the container body, so that the ratio of the material thickness d ofthe container body to the material thickness d₁ of the neck portion isapproximately equal to the ratio of the radius r₁ of the neck to theradius r of the container body. The neck consists of axially orientedmaterial and the container body of biaxially oriented material.

In a preferred embodiment, the central material portion 24 and/or theorifice edge 22 consist of thermocrystallized, opaque, dimensionallystable material.

In employing the invention, a tubular blank is stretched in its axialdirection by passing it through one or more draw rings, which reduce thewall thickness of the blank and at the same time diminish the outercircumference of the blank. Immediately prior to stretching, thematerial of the blank is at a temperature within or preferably below therange of the glass transition temperature (TG) of the material.

The preform 20, produced by stretching the blank, is then, after havingbeen heated to the moulding temperature, introduced into the blow mold12. The mandrel 15 is introduced into its position inside the preform,thereby holding the orifice of the preform 22 against the grippingdevices 13a and 13b. The base portion 11 of the blow mold 12 is in itslower position. The heating medium passes through the channels 18, 19,30, 31 so as to bring to temperature the adjacent shaping surfaces ofthe blow mold and above all to heat up the surfaces adjoining thegripping devices 13a 13b around the orifice 22 of the preform.

Thereafter, the interior of the preform is placed under pressure bymeans of a pressure medium at the same time as the base portion 11 ofthe blow mold is moved towards the orifice of the preform, that is tosay upwards in the figures. As a result, the preform expands at the sametime that its axial length is reduced (see FIG. 2), so that the materialof the preform is stretched essentially only in the circumferentialdirection of the preform i.e. there is no axial stretching or strain ofthe preform. When the base portion of the blow mold has reached itsupper position (FIG. 3), all portions of the material of the preformhave also been brought into contact with the inner shaping surfaces ofthe blow mold by means of the pressure medium, and the preform has beenconverted to the container 21.

In certain embodiments, the inner pressure in the container ismaintained for some time so that reliable contact with the blow mold isachieved. The shaping surfaces of the blow mold are then at atemperature in the range of between 110° and 180° C., preferably 130°and 150° C., as a result of which possible stresses in the material arereleased through contact and at the same time a certain amount ofthermal crystallization takes place in the material. This mechanicallystabilizes the shape of the container, and the latter can, without majorchange of shape, be reheated to the temperature at which theheat-stabilization took place.

When the heat-stabilization of the container is finished, the mouldhalves are opened, the base portion of the blow mold is moved to itslower position and the produced container is taken out of the blowmould.

In certain applications, there occurs cooling of the material of thepreform and hence of the central base portion of the container, i.e. ofthe material which in this case is reshaped, and/or reduced inthickness, between the shaping surface 16 of the mandrel 15 and thecentral mould portion 17 of the base portion 11 of the blow mold. Thisgives a container whose central base portion consists of amorphousmaterial. In other applications, the material is heated as justdescribed in connection with the heat-stabilization of the container,with the shaping surface 16 of the mandrel 15 having a temperaturewithin the range at which the amorphous material will cristallize,preferably a temperature of 130°-160° C. In this way, the central baseportion of the container is converted to a thermo-crystallized, opaque,dimensionally stable material.

In certain applications, where the requirement for dimensional stabilityof the moulded container at elevated temperatures is not as great, thematerial in the preform is stretched, during moulding, also in the axialdirection of the preform. The stretching is however relatively slightand in the case of PET must not be equivalent to more than about 30%lengthening of the material. The axial stretching ratio is controlled,according to the invention, by the amount of the vertical movement ofthe base portion 11 of the blow mold.

The invention has been described above in connection with a blow moldwhose base portion travels in the axial direction of the blow moldduring the conversion of the preform to a container. It is obvious thatthe invention is not restricted to the design described, but that theconcept of the invention as such can also be realized, for example, bymeans of a blow mould which has a fixed base portion, and in which thegripping halves, together with the mandrel, are moved in the axialdirection of the blow mold during the conversion of the preform.

A container according to the invention has a crystallinity, in the neckportion 25, of the order of 10-30% and, in the container body 26, of theorder of about 10-40%. In the embodiment where the central shapingportion 17 keeps the central base portion 24 at a temperature which doesnot cause thermally conditioned crystallization, or causes thiscrystallization only to a slight extent (such a temperature being, forPET, below about 100° C.), an amorphous central base portion 24, with acrystallinity of less than 10% preferably less than 5%, is obtained. Inthe case where the central shaping portion 17 holds the material at thecristallization temperature (about 140° C. for PET), a cristallization,opaque, extremely dimensionally stable central area of material, with acrystallinity in excess of about 10%, is obtained. Similar conditionsregarding crystallinity apply to the edge of the orifice 22, dependingon whether it has been cooled or heated to the temperature regions justmentioned, by the mandrel 15 and/or gripping devices 13 a, b.

The crystallinity values stated in the present application relate to thetheories disclosed in the publication "Die Makromolekulare Chemie" 176,2459-2465 (1975).

The invention will be clear not only from the above description but itsscope and extent will become evident from the claims which follow.

What is claimed is:
 1. A method of forming a container from orientablethermoplastic material comprising:providing a hollow preform ofthermoplastic material including a side wall and a closed end, thematerial of said side wall having axial orientation and being capable ofcircumferential expansion and circumferential orientation, positioningthe preform in a mold having a movable mold part, expanding the preformin the mold to form the container, said preform, prior to its formationto the finished shape of said container, coming into contact with saidmovable mold part without any axial stretching of said side wall, anddisplacing the closed end portion of the preform by said movable moldpart in correspondence with the expanding of the preform such that thematerial of the side wall is not stretched in a plane perpendicular tothe direction of expanding and thereby the material does not undergo anyaxial strain due to elongation and said expanding producescircumferential stretching and circumferential orientation of thematerial in said side wall independent of and superimposed on said axialorientation.
 2. A method as claimed in claim 1 wherein said preform isprogressively expanded in correspondence with progressive displacementof the closed end of the preform.
 3. A method as claimed in claim 1comprising effecting said expanding with the temperature of the materialin the vicinity of the glass transition temperature (T_(g)).
 4. A methodas claimed in claim 3 wherein the axial orientation of the material ofthe preform is effected by stretching the material in the axialdirection at a temperature within or below the region of the glasstransition temperature (T_(g)) of the material.
 5. A method as claimedin claim 4 wherein said preform is tubular and is stretched in its axialdirection to reduce its wall thickness while concurrently diminishingthe outer circumference of the preform.
 6. A method as claimed in claim5 wherein the inner diameter of the preform is substantially maintainedduring the stretching of the blank.
 7. A method as claimed in claim 4wherein said preform is tubular and has inner and outer surfaces, saidpreform being stretched in its axial direction to reduce the wallthickness while substantially maintaining the diameter of one of saidsurfaces and changing the diameter of the other of said surfaces.
 8. Amethod as claimed in claim 1 comprising heating said preform, prior toplacement in the mold, to a temperature above the glass transitiontemperature (T_(g)).
 9. A method as claimed in claim 1 wherein thepreform is at a molding temperature prior to expanding.
 10. A method asclaimed in claim 1 wherein the side wall of the preform is out ofcontact with the mold when the closed end of the preform contacts saidmovable mold part.
 11. A method as claimed in claim 1 wherein thepreform comes into contact with said movable mold part before thepreform is expanded.
 12. A method as claimed in claim 11 wherein saidexpanding is effected by blowing under pressure.
 13. A method as claimedin claim 1 wherein the circumferential stretching of the side wallcauses reduction in the thickness thereof such that the resultantthickness of the side wall is inversely related to its diameter.
 14. Amethod as claimed in claim 1 wherein said closed end of said preform iscompressed against said movable mold part to undergo shaping andthickness change.
 15. A method as claimed in claim 1 wherein said moldis heated and said container undergoes heat transfer with the mold andconsequent heat setting.
 16. A method as claimed in claim 1 whereinduring the expanding of the preform to said container, said moveablemold part is moved in the axial direction of the preform, to assume anupper position corresponding to the final shape of the mold and therebyalso to the final shape of the container produced in the mold.
 17. Amethod as claimed in claim 16 wherein a central portion of the closedend of the preform is reduced in thickness by compressing the materialin said central portion between the movable mold part and a mandrellocated inside the preform.
 18. A method as claimed in claim 17 whereinsaid preform is expanded to said container by applying pressure insidethe preform to expand the same against the walls of the mold, saidmethod further comprising supplying heat to the preform from said wallsso that the material of the container, through heat transmission fromthe mold, acquires an additional crystallinity over and above that whichresults from the orientation of the material while additionally internalstresses of the material originating from the orientation are released.19. A method as claimed in claim 1 wherein said thermoplastic materialis polyethylene terephthalate, polyhexamethylene adipamide,polycaprolactam, polyhexamethylene sebacamide, polyethylene 2,6- and1,5- naphthalate, polytetramethylene 1,2-dioxybenzoate or copolymers ofethylene terephthalate and ethylene isophthalate.
 20. A method asclaimed in claim 4 wherein the material has been stretched to undergoflow.
 21. A method as claimed in claim 4 wherein said thermoplasticmaterial is polyethylene terephthalate and said preform is stretched atleast about 3-fold.
 22. A method as claimed in claim 1 wherein saidthermoplastic material is polyethylene terephthalate and said preform isproduced by stretching a blank at a temperature below 105°.
 23. A methodas claimed in claim 22 wherein the temperature of the material atstretching is between 70° and 105° C.